1. Field of the Invention
The present invention relates to a microstructure manufacturing method and a microstructure, and, in particular, to a method of manufacturing an absorption grating of an X-ray imaging apparatus and an absorption grating.
2. Description of the Related Art
A diffraction grating consisting of a microstructure having a periodic structure is utilized in various apparatuses as a spectral element. A microstructure formed of gold utilizing the absorption characteristics of an X-ray is used in a nondestructive test of an object as an industrial application and in radiography as a medical application. In these uses, a contrast image is formed by utilizing a difference in absorption at the time of X-ray transmission due to the constituent elements and a difference in density in an object or a biological object, and this technique is referred to as an X-ray absorption contrast method.
Also in the Talbot interferometry, which is one of the phase contrast methods using a phase difference in an X-ray, there is used an absorption grating formed of gold, which has a periodic structure of large X-ray absorption. As a method of preparing an absorption grating formed of gold having periodic structure, a method in which a mold is filled with gold by plating is preferable.
The literature: “Microelectronic Engineering” Vol. 84, 1172 (2007), discusses a method using a one-dimensional mold formed through anisotropic wet etching of silicon. According to this method, an aluminum layer is formed on an apex portion of a one-dimensional mold by oblique deposition. Subsequently, gold is deposited on the bottom portion of the one-dimensional mold and on the aluminum layer by a deposition method with directivity. Then, the gold on the aluminum layer is also removed at the time of removal of the aluminum layer through etching, and, thereafter, an absorption grating formed of a gold-cobalt alloy is prepared using a cyan-type gold-cobalt plating solution.
In the above literature, the absorption grating is flat, so that it is effective in treating parallel rays, for example, in the case of a radiation facility. However, in an imaging in a laboratory using an X-ray source in the form of a point light source of an X-ray tube, the advancing direction of the X-ray and the depth direction of the grating are deviated from each other near the periphery of the X-ray absorption grating. Furthermore, if a protrusion forming the absorption grating is not situated at one point where the light is condensed, an X-ray to be transmitted through the absorption grating can instead be shielded.